Distored image when convert RGB into YUV. How to fix???

From .ppm image, I extracted R,G,B data and saved following structures:

RGB RGB RGB RGB RGB RGB RGB..........

corresponding to each pixel position.
Then I used formula to convert R,G,V into Y,U,V.With each pixel, I obtained YUV correspondingly.

R0G0B0->Y0U0V0 , R1G1B1 ->Y1U1B1, ........

I saved data following YUV422 Data Format:The YUV422 data format shares U and V values between two pixels. As a result, these values are transmitted to the PC image buffer only once for every two pixels.

R0G0B0R1G1B1->Y0UY1V

a,How to calculate U and V from U0,U1 and V0,V1????????
b,In my case , I used this formula:

U=(U0+U1)/2; V=(V0+V1)/2;

Obtained data was saved following structure to create .yuv file:

YUYV YUYV YUYV YUYV YUYV YUYV......

But when I used YUV tools to read new .yuv file, that image is not similar to original image. What did I do wrong here ????

This thread should be moved to tech board